Preloads fail silently when given incorrect query

I gave a preload an incorrect query, in an ill-considered attempt to optimize:
```crystal
preload_locations(LocationQuery.new.preload_url_path(URLPathQuery.new.limit(1))
```
The `limit(1)` is incorrect, `preload_locations` will retrieve more than one object. The result was an object with `@_url_path_preloaded=true`, and `@_preloaded_url_path=nil`, and no complaint from the preload code.

Start by tracing preload_locations and how it handles a query with limit(1), using the example in this issue to reproduce the silent failure. Done means an incorrect preload query reports a complaint instead of producing a preloaded flag with a nil value.

Written by the indexing model from the issue text.